Abstract: A probabilistic method for the stability of shield tunnelling face is proposed based on the concept of stability ratio defined by B. B. Broms and H. Bennermark in consideration of the spatial variability of soil parameters. The undrained shear strength of clay is regarded as a stochastic variable following lognormal distribution, and the failure probability of excavation face affected by the linear increases of variation coefficient and mean value of undrained shear strength against depth is analyzed. Results indicate that the spatial variability of undrained shear strength has a significant effect on face stability. The probability of failure increases correspondingly as coefficient of variation increases, but decreases with the growing of linear coefficient of the mean undrained shear strength. Probabilistic method is more suitable in evaluating the face stability of shield tunnelling than single face stability ratio.

Key words: tunnel, face stability, stability ratio, spatial variability, probabilistic method